Formative assessments are really where you have the chance to catch misconceptions and make interventions before it's too late. But for some reason, until last year, I never put as much emphasis or attention on my quiz data. If you aren't looking at assessment data, you are missing a HUGE component of what can make a difference in the success of your students and you as an educator. Data should drive the instructional decisions you make. We don't have the luxury of tons of extra time, so why waste your precious minutes on assumptions about what you think your students are struggling with and what they have mastered.